1. Field of the Invention
The present invention generally relates to optical recording media for storing a variety of information, such as image information and audio information, an apparatus for reproducing the information recorded on such optical recording media, and a method of recording and reproducing the information recorded on such optical recording media.
2. Description of the Related Art
Optical recording media, such as optical-disks, are used for recording a large amount of information. An optical disk has a center hole formed at the center thereof, and a plurality of turns of a track spirally formed on a circumferential recording area of the optical disk.
As shown in FIG. 1A, an optical disk has a transparent disk substrate 15 made of, for example, PMMA (PolyMethyl MethAcrylate), and information pits 10 are formed on the transparent disk substrate 15 so that they project therefrom. Information pits 10 correspond to information recorded on the optical disk. An aluminum film 12 having a high reflection rate is vapor-deposited on the disk substrate 15 on which the information pits 10 are formed. A protection film 11 made of, for example, plastics is formed on the aluminum film 12.
The information recorded on the optical substrate 15 is reproduced as follows. A light beam LB, such as a laser beam, is projected onto the optical disk from the side of the disk substrate 15 so that the light beam LB is projected onto the track. As shown in FIG. 1B, a light beam LB.sub.R reflected by a mirror plane portion between adjacent pits is detected as "a bright light". The information pits 10 irregularly reflect the light, and change the reflection rate so that the amount of the reflected light beam LB.sub.R decreases. Thus, as shown in FIG. 1B, the reflected beam LB.sub.R obtained by projecting the light beam LB onto an information pit portion 2 is detected as "a dim light". As shown in FIG. 1C, information corresponding to recording pits 20a, 20b, 21 . . . of the information pit portion 2 can be reproduced by detecting the "bright" and "dim" reflected lights.
As described above, the recording of information is based on the presence/non-presence of the information pits 10, which modulate the amplitude (intensity) of the reflected light used for reading information recorded on the optical disk. The reproduction of information is carried out by detecting changes in the amount of the reflected light which has been amplitude-modulated by the information pits 10. The presence/non-presence of an information pit corresponds to a one-bit binary signal. In principle, it is possible to increase the recording density by decreasing the distance between adjacent information pits. As the distance between adjacent information pits decreases, it is necessary to reduce the size of the light spot projected on the optical disk. If the light spot does not have a sufficiently reduced size, adjacent information pits will be simultaneously illuminated, as shown in FIG. 1C. This prevents information from being accurately read from the optical disk. Hence, the optical disk as shown in FIG. 1A has a limited information recording density per unit area.
Japanese Laid-Open Patent Publication No. 50-119517 discloses a recording carrier (recording medium) intended to overcome the above problem. The recording medium disclosed in the above publication has a track area in which at least two types of diffraction lattices having respective lattice lines oriented in different directions are provided. When one track is read, this track serves as an intermediate area between adjacent tracks located on respective sides thereof. With this arrangement, it becomes possible to obtain a recording density approximately twice that of the optical disk shown in FIG. 1A, with respect to a light spot of same size.
However, the recording carrier disclosed in the above mentioned publication has a disadvantage in that a plurality of signal detectors are needed because a plurality of types of diffraction lattices respectively having different directions of diffraction are used. In addition, the recording carrier disclosed in the publication has a recording density increased to be approximately twice that of the optical disk shown in FIG. 1A, and thus cannot provide a drastically increased recording density.